A pneumatic device, such as a pneumatic cylinder, is supplied with air from a pneumatic source via a pneumatic line such as piping or hose. By connecting the pneumatic source and the pneumatic device via the pneumatic line, a pneumatic circuit is formed. Air to be supplied from the pneumatic source to the pneumatic device is handled as air to be treated, and the pneumatic circuit is provided with a filter for removing foreign matter, such as water droplets or oil droplets or dusts, included in the air to be treated.
As one type of the filter provided in the pneumatic circuit, Japanese Patent Laid-Open Publication No. H07-328364 discloses a filter having a main block, that is, a port block formed with a primary port and a secondary port, and a filter element attached to the port block. The filter element is adapted to remove foreign matters from air flowing through the primary port, the matter being composed of liquid droplets such as water droplets, and dust such as powdery and granular materials, and to discharge the purified air to the secondary port. In order to receive the foreign matter, such as liquid droplets, removed by the filter element, a filter bowl, namely, a collection container is attached to the port block.
As a filter to be used in the pneumatic circuit, forms called air filter, mist filter, and micro mist filter are known, and these filters are defined according to foreign matter removal performance set on the basis of an inner diameter of an air hole of a filter element or the like.
A filter adapted to swirl liquid to remove foreign matter mixed in liquid coolant is disclosed in Japanese Patent Laid-Open Publication No. 2011-51055. This filter is adapted to swirl liquid to remove foreign matter from liquid on the basis of the difference in specific gravity and centrifugal force between the liquid and foreign matter.
In a filter adapted to swirl air in a separation cylinder to utilize the difference in centrifugal force between air and foreign matter, in order to remove foreign matter such as liquid droplets or dust from air, the foreign matter falls down along an inner peripheral surface of the separation cylinder, while air purified by removing the foreign matter is supplied outside via an exhaust pipe disposed in the center of the cylinder.
In the filter adapted to swirl air to fall liquid droplets or the like in air into the collection container as just described, foreign matter such as liquid droplets fall down into the collection container via an exhaust port at a lower end portion of the separation cylinder, while being guided along the inner peripheral surface of the separation cylinder. If the inner peripheral surface of the separation cylinder has a conical shape and its inner diameter is gradually reduced toward the lower end portion of the separation cylinder, in particular, if a lot of liquid droplets are contained in air supplied from outside, foreign matter removal efficiency tends to be reduced.
In pursuit of the cause of the efficiency reduction, it was found that a bridge phenomenon may occur due to condensation of liquid droplets falling along a conical inner peripheral surface in the vicinity of the exhaust port. That is, when the pressure in a storage chamber of the collection container becomes higher than that of a separation chamber of the separation cylinder by a rapid drop or the like in secondary side pressure, a membrane is formed by condensation of liquid droplets in the vicinity of the exhaust port. Since pushing-up force due to the pressure in the storage chamber acts on this water membrane, the water membrane shuts the exhaust port without falling down. When this bridge phenomenon occurs, liquid shuts the exhaust port without falling down by its own weight into the collection container via the exhaust port, and liquid is mixed with air flowing toward the exhaust pipe and discharged via the secondary port, so that such a bridge phenomenon causes a reduction in foreign matter removal efficiency.
An object of the present invention is to remove foreign matter such as liquid droplets with high efficiency from air by using a filter.